The present invention relates to a turbine bucket for a gas turbine stage and particularly relates to a second stage turbine bucket airfoil profile.
There are many considerations in the design and construction of turbine buckets, particularly their airfoils, including optimized aerodynamic efficiency and aerodynamic and mechanical bucket loading. Additionally, bucket airfoil design must also take into consideration the potential mismatch or engagement problems associated with bucket airfoils having tip shrouds. As will be appreciated, certain buckets in turbines are provided with bucket tip shrouds which circumferentially engage one another along leading and trailing edges in a circumferential direction. Typically, the shrouds mount a seal which cooperates with a fixed shroud to seal against hot gas bypass between high and lower pressure regions on opposite sides of the bucket airfoils. The shrouds are also provided on long and slender buckets to add stiffness to the bucket airfoils by the engagement of the shrouds with one another. However, with air-cooled buckets, differential thermal growth and twisting sometimes affords poor engagement of the shrouds with one another. That is, one edge of the shroud may be radially inwardly of the opposing edge of the adjacent shroud. Absent an ideal engagement between adjacent shrouds, adverse loading causes higher stress at points of contact. With loss or minimization of contact, the benefit of damping vibrations to avoid high cycle fatigue by using shrouds is minimized or lost. Less than optimum tip shroud engagement adversely impacts tip shroud creep life and reduces part life. It will also be appreciated that the failure of a single bucket including its airfoil causes the entire turbine to be taken offline. These are time-consuming and expensive repairs which include the cost of the outage to the user of the turbine.
In accordance with a preferred embodiment of the present invention, there is provided a unique turbine bucket airfoil profile, preferably for air-cooled tip shrouded airfoils of the second stage of a gas turbine. The bucket airfoil profile yields substantially improved shroud-to-shroud engagement enabling significant increased part life and reduced repair costs. Additionally, the airfoil reduces local creep and affords improved HCF margin in the resulting airfoil. The bucket airfoil profile is defined by a unique loci of points to achieve the necessary efficiency, loading and tip shroud engagement requirements. These unique loci of points define the nominal airfoil profile ranging from 10-90% span of the airfoil height and are identified by the X, Y and Z Cartesian coordinates of Table I which follows. The points for the coordinate values shown in Table I are for a cold, i.e., room temperature profile at various cross-sections of the bucket airfoil within the 10-90% span of the airfoil height. The X, Y and Z coordinates are given in distance dimensions, e.g., units of inches. The X and Y coordinate values are joined smoothly with one another at each Z location to form smooth continuous arcuate airfoil profile sections. The Z coordinates are distances from and perpendicular to a plane passing through a turbine axis of rotation. Each defined airfoil profile section at each Z distance is joined smoothly with adjacent airfoil profile sections to form the complete airfoil shape.
It will be appreciated that as each bucket airfoil heats up in use, the profile will change as a result of stress and temperature. Thus, the cold or room temperature profile is given by the X, Y and Z coordinates for manufacturing purposes. Because a manufactured bucket airfoil profile may be different from the nominal airfoil profile given by the following table, a distance of plus or minus 0.016 inches from the nominal profile in a direction normal to any surface location along the nominal profile and which includes any coating process, defines the profile envelope for this bucket airfoil. The design is robust to this variation without impairment of the mechanical and aerodynamic functions.
It will also be appreciated that the airfoil can be scaled up or scaled down geometrically for introduction into similar turbine designs. Consequently, the X and Y coordinates in inches of the nominal airfoil profile given below are a function of the same constant or number. That is, the X and Y, and optionally the Z, coordinate values in inches may be multiplied or divided by the same constant or number to provide a scaled up or scaled down version of the bucket airfoil profile while retaining the airfoil section shape.
In a preferred embodiment according to the present invention, there is provided a turbine bucket including a bucket airfoil having an airfoil shape, the airfoil having nominal profile ranging from 10-90% span of the airfoil height substantially in accordance with Cartesian coordinate values of X, Y and Z set forth in Table I wherein Z is a distance in inches from and perpendicular to a plane passing through an axis of rotation of the turbine and wherein X and Y are distances in inches which, when connected by smooth continuing arcs, define airfoil profile sections at each distance Z, the profile sections at the Z distance being joined smoothly with one another to form the complete airfoil shape.
In a further preferred embodiment according to the present invention, there is provided a turbine bucket including a bucket airfoil having an airfoil shape, the airfoil having an uncoated nominal airfoil profile ranging from 10-90% span of the airfoil height substantially in accordance with Cartesian coordinate values of X, Y and Z set forth in Table I wherein Z is a distance in inches from and perpendicular to a plane passing through an axis of rotation of the turbine and wherein X and Y are distances in inches which, when connected by smooth continuing arcs, define airfoil profile sections at each distance Z, the profile sections at the Z distances being joined smoothly with one another to form the complete airfoil shape, the X and Y distances being scalable as a function of the same constant to provide a scaled-up or scaled-down bucket airfoil.
In a further preferred embodiment according to the present invention, there is provided a turbine comprising a turbine wheel having a plurality of buckets, each of the buckets including a bucket airfoil having an airfoil shape, the airfoil having a nominal profile ranging from 10-90% span of the airfoil height substantially in accordance with Cartesian coordinate values of X, Y and Z set forth in Table I wherein Z is a distance in inches from and perpendicular to a plane passing through an axis of rotation of the turbine axis and wherein X and Y are distances in inches which, when connected by smooth continuous arcs, define airfoil profile sections at each distance Z, the profile sections at the Z distances being joined smoothly with one another to form the complete airfoil shape.
In a further preferred embodiment according to the present invention, there is provided a turbine comprising a turbine wheel having a plurality of buckets, each of the buckets including a bucket airfoil having an airfoil shape, the airfoil having a nominal profile ranging from 10-90% span of the airfoil height substantially in accordance with Cartesian coordinate values of X, Y and Z set forth in Table I wherein Z is a distance in inches from and perpendicular to a plane passing through an axis of rotation of the turbine and wherein X and Y are distances in inches which, when connected by smooth continuous arcs, define airfoil profile sections at each distance Z, the profile sections at the Z distances being joined smoothly with one another to form the complete airfoil shape, the X and Y distances being scalable as a function of the same constant to provide a scaled-up or scaled-down bucket airfoil.